In recent years, in the cellular mobile communication system, a transmission of large capacity data such as still image data and motion image data as well as audio data has become generalized in accordance with the expansion of multimedia of information. In order to realize the transmission of large capacity data, a technique for realizing the high transmission rate utilizing a high-frequency radio band has been investigated flourishingly.
At the time of utilizing the high-frequency radio band, however, although the high transmission rate can be expected at a short distance, an amount of attenuation due to the transmission distance increases in accordance with the increase of the distance. Thus, in the case of actually employing the mobile communication system utilizing the high-frequency radio band, the coverage area of a wireless communication base station apparatus (Evolved NodeB: eNB, hereinafter referred to as a base station) becomes small. Therefore, it becomes necessary to install a lot of base stations. Since it takes a correspondence cost at the time of installing the base station, a technique has been demanded strongly which can realize the communication service utilizing the high-frequency radio band while suppressing the increase of the number of the base stations.
According to the aforesaid demand, in order to enlarge the coverage area of each of the base stations, the investigation has been made as to a relay transmission technique in which, as shown by a wireless relay system of a related art shown in FIG. 15, a wireless communication relay station apparatus (hereinafter referred to as a relay station) 30 is provided between a base station 10 and a wireless communication terminal apparatus (User Equipment: UE, hereinafter referred to as a terminal) 50B to thereby perform communication between the base station 10 and the terminal 50B via the relay station 30. By employing the relaying technique, the terminal 50B, that can not directly perform the communication with the base station 10, can perform the communication via the relay station 30. A terminal 50A is connected to the base station 10 and hence can directly perform the communication with the base station 10.
[Explanation of TD Relay]
The TD relay is a system for dividing communication in a backhaul link and an access link in a time-sharing manner. According to the TD relay, the relay station can perform reception via the backhaul link and transmission via the access link without being influenced by unintended loop-back between the transmission antenna and the reception antenna of the relay station. When the TD relay is applied, however, there arises a period during which the relay station stops the transmission to the access link in the backhaul downlink. The LTE (Long Term Evolution; hereinafter referred to as LTE) terminal operates on the assumption that the base station periodically transmits a reference signal via the downlink. Thus, when the relay station stops the transmission of data including the reference signal and the like in a subframe, there arises a problem that the terminal erroneously detects the reference signal.
[Utilization of MBSFN Subframe]
The LTE-A (Long Term Evolution Advanced) system is required to maintain the compatibility with the LTE in view of the smooth transition from the LTE and the coexistence with the LTE. Thus, this system is required to attain the mutual compatibility with the LTE also as to the Relay technique. In the LTE-A system, in order to attain the mutual compatibility with the LTE, it has been investigated to set an MBSFN subframe at the time of transmission to the relay station from the base station in a downlink (hereinafter referred to DL).
The “MBSFN subframe” is a subframe defined in order to transmit MBMS (Multimedia Broadcast Multicast Service) data. The LTE terminal is defined in its operation so as not to use the reference signal in the MBSFN subframe.
Thus, in the LTE-A, there has been proposed a method that, in a subframe for the backhaul link where the relay station 30 performs the communication with the base station 10, the subframe on the access link side of an RN (relay station) cell is set to the MBSFN subframe to thereby avoid the erroneous detection of the reference signal at the LTE terminal. FIG. 16 shows an example of the allocation of a control signal and data in the subframe in each of the respective stations in the LTE system. As shown in FIG. 16, in the LTE system, the control signal is arranged at the head of the subframe in each of the respective stations. Thus, since the relay station 30 must transmit the control signal portion to the terminal 50B even in the MBSFN subframe, the relay station 30 changes its mode into a reception mode after transmitting the control signal to the terminal 50B and receives the signal from the base station 10. Accordingly, the relay station 30 can not receive the control signal transmitted from the base station 10. As a result, in the LTE-A, it has been investigated to newly arrange the control signal for the relay station 30 into the data area.
[Explanation of Control Signal]
The control signal of the LTE system is transmitted to the terminal from the base station by using a downlink control channel such as a PDCCH (Physical Downlink Control Channel). Each of the PDCCHs is arranged in one or plural CCEs (Control Channel Elements), that is, logical resource.
In the case where one PDCCH is arranged in one or plural CCEs, the one PDCCH is arranged in continuous plural CCEs.
The CCEs where the respective PDCCHs are arranged are mapped on corresponding REGs (Resource Element Groups) as physical resources. The one CCE is mapped on 9 REGs. The REG is configured by 4 REs. The RE represents a resource unit of (1 subcarrier*1 OFDM symbol).
[Example of RB for Control Signal for Relay Station (R-PDCCH) (4 Antennas)]
Explanation will be made as to an example (4 antennas) of the resource block (hereinafter referred to RB) for the control signal for the relay station with reference to FIG. 17. It is supposed that 1 RB is configured by (12 subcarriers×14 OFDMs). A block of minimum unit drawn by a (thin) solid line in FIG. 17 represents 1 RE. A block drawn by a (thick) solid line in FIG. 17 represents 1 REG (configured by 4 REs). Of the blocks of the minimum unit drawn by the (thin) solid lines in FIG. 17, the block shown by Rn (n=0 to 3) represents the RS of the n-th antenna n. It is supposed that 1 CCE is configured by 9 REGs.
In the example of 1 resource block (RB) shown in FIG. 17, since 1 RB is configured by (12 subcarriers×14 OFDMs), 168 REs can be arranged per 1 RB. Further, as shown in FIG. 17, when 24 REs are used for the transmission of the RSs (R0, R1, R2, R3) and the latter 11 OFDM symbols are used for the R-PDCCH, 116 REs can be used for the R-PDCC.
Like the LTE, in the 1 resource block (RB) shown in FIG. 17, since the 1 REG is configured by 4 REs, 29 REGs can be used for the R-PDCCH. Further, in the 1 resource block (RB) shown in FIG. 17, since the 1 CCE is configured by the 9 REGs, 3 CCEs (9*3=27 REGs) can be allocated for the 1 RB. Thus, there are two unused REGs obtained by subtracting 27 REGs from 29 REGs usable as the R-PDCC.